Field-angle dependence of sound velocity in the Weyl semimetal TaAs

TL;DR

This study investigates how the sound velocity in TaAs varies with magnetic field orientation, revealing symmetry breaking, quantum oscillations, and topological signatures related to Weyl fermions and the chiral anomaly.

Contribution

It provides the first detailed analysis of field-angle dependence of sound velocity in TaAs, linking elastic response to topological electronic properties.

Findings

Broken tetragonal symmetry above 6 T

Quantum oscillations with orientation-dependent frequencies

Detection of topological and trivial Berry phases

Abstract

The elastic modulus $c_{44}$ of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. We also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes respectively.